The present invention relates to an information processing system including a storage subsystem, and particularly to a technique for transferring data stored in storage subsystems between plural information processing systems.
In an information processing system including a storage subsystem, in the case where a failure occurs in the storage subsystem due to a power failure, a natural disaster or the like, business operations using the information processing system are temporarily stopped, and in the worst case, data stored in the storage subsystem are lost. In order to avoid such a situation, there is a technique (hereinafter referred to as a “remote copy”) for transferring and copying data stored in the storage subsystem of the information processing system to a storage subsystem provided at a remote place different from the information processing system.
The remote copy includes two kinds of remote copies, that is, a synchronous remote copy and an asynchronous remote copy, and they have merits and demerits, respectively. Specifically, in the synchronous remote copy, in the case where a write request is issued from a computer of an information processing system, after completing transfer of data accompanying the write request to a storage subsystem existing at a remote place, a storage subsystem of the information processing system makes a response to the computer as to the write request. Accordingly, in the synchronous remote copy, although data loss due to the failure is small, when a line delay between the storage subsystems is increased, the I/O performance between the computer and the storage subsystem deteriorates.
On the other hand, in the asynchronous remote copy, the storage subsystem of the information processing system executes the response to the computer as to the write request and the transfer of the data accompanying the write request to the remote place at independent timings. Accordingly, in the asynchronous remote copy, even if the distance between the storage subsystems is long, the performance is hardly lowered, however, the possibility of data loss becomes higher than the synchronous remote copy.
In recent years, in order to compensate the demerits of both remote copies, a remote-copy technique using plural information processing systems (hereinafter referred to as “sites”) starts to be used.
For example, U.S. Pat. No. 6,209,002 discloses a system including a second site (including a second storage subsystem) near a storage subsystem included in a first site, and a third site (including a third storage subsystem) remote from the first site. In this system, two modes are alternately carried out.
Specifically, in a first mode, the synchronous remote copy is performed from the first storage subsystem to the second storage subsystem, and the remote-copied data are duplicated in the second storage subsystem. Incidentally, in this mode, a remote copy from the second storage subsystem to the third storage subsystem is not performed.
On the other hand, in a second mode, while the synchronous remote copy from the first storage subsystem to the second storage subsystem is continued, the asynchronous remote copy from the second storage subsystem to the third storage subsystem is performed. However, in this mode, the duplication of the data in the second storage subsystem is stopped.
Besides, “Addressing Federal Government Disaster Recovery Requirements with Hitachi Freedom Storage” by Claus Mikkelsen et al. also discloses a system including a second site (including a second storage subsystem) near a storage subsystem included in a first site, and a third site (including a third storage subsystem) remote from the first site. In this system, in order to perform a copy from the first storage subsystem to the third storage subsystem, two modes are alternately carried out.
Specifically, the synchronous remote copy is always performed from the first storage subsystem to the second storage subsystem. Besides, in a first mode, data written by a computer of the first site is duplicated in the first storage subsystem. Incidentally, in this mode, the remote copy from the first storage subsystem to the third storage subsystem is not performed.
On the other hand, in a second mode, the asynchronous remote copy from the first storage subsystem to the third storage subsystem is performed. However, in this mode, the duplication of the data in the first storage subsystem is stopped.
In the techniques disclosed in U.S. Pat. No. 6,209,002 and “Addressing Federal Government Disaster Recovery Requirements with Hitachi Freedom Storage” by Claus Mikkelsen et al., since the copy of data to the third storage subsystem is only periodically performed, there is a possibility that data lost becomes large, in the case where the first storage subsystem and the second storage subsystem simultaneously fail.